Polymetallic or manganese nodules, are rock concretions formed of concentric layers of iron and manganese hydroxides around a core.
Seafloor nodules on the ocean floor include manganese (Mn) in their composition, and usually contain Ni, Co, Cu, Zn, and Fe, with minor amounts of titanium, vanadium, molybdenum, and cerium. Often present in addition are one or more of the following metals: magnesium, aluminum, calcium, cadmium, potassium, sodium, zirconium, titanium, lead, phosphorus, and barium. Deep sea nodules generally contain about 28% Mn, about 10% FeO, about 1% Cu, and about 1.2% Ni.
Most of the desired valuable metals in manganese nodules are tied up with insoluble oxidized manganese, such as MnO2. Less than 10% of the manganese contained in the nodules is acid soluble. Thus it is necessary to reduce the MnO2 by a suitable reducing agent as a first step in order to recover the metal constituents. Historically, SO2 has been used for this purpose, and carbon monoxide (CO) has been used in copper recovery processes. However such prior art processes often do not recover a suitable manganese product and are capable of recovering only from about 80 to about 92% of the primary metal values, and often produce large quantities of waste, which may include metal components that are not completely removed.